Trunked RF repeater systems have become a mainstay of modern RF communications systems, and are used, for example, by public service organizations (e.g., governmental entities such as counties, fire departments, police departments, etc.). Such RF repeater systems permit a relatively limited number of RF communications channels to be shared by a large number of users--while providing relative privacy to any particular RF communication (conversation). Typical state-of-the-art RF repeater systems are "digitally trunked" and use digital signals conveyed over the RF channels (in conjunction with digital control elements connected in the system) to accomplish "trunking" (time-sharing) of the limited number of RF channels among a large number of users. A number of such systems are commercially available, such as the Clearchannel LTR system from E. F. Johnson and the Smartnet and Privacy Plus systems from Motorola.
Briefly, such digitally trunked RF communications systems include a "control" RF channel and multiple "working" RF channels. The working channels are used to carry actual communications traffic (e.g., analog FM, digitized voice, digital dam, etc.). The RF control channel is used to carry digital control signals between the repeater sites and user RF transceivers (radio units) in the field. When a user's transceiver is not actively engaged in a conversation, it monitors the control channel for "outbound" digital control messages directed to it. User depression of a push-to-talk (PTT) switch results in a digital channel request message requesting a working channel (and specifying one or a group of callees) to be transmitted "inbound" over the RF control channel to the repeater site. The repeater site (and associated trunking system) receives and processes the channel request message.
Assuming a working channel is available, the repeater site generates and transmits a responsive "outbound" channel assignment digital message over the RF control channel. This message temporarily assigns the available working channel for use by the requesting transceiver and other callee transceivers specified by the channel request message. The channel assignment message automatically directs the requesting (calling) transceiver and callee transceivers to the available RF working channel for a communications exchange.
When the communication terminates, the transceivers "release" the temporarily assigned working channel and return to monitoring the RF control channel. The working channel is thus available for reassignment to the same or different user transceivers via further messages conveyed over the RF control channel. An exemplary "single site" trunked RF repeater system of this type is disclosed in commonly-assigned U.S. Pat. Nos. 4,905,302 and 4,903,321.
Single site trunked RF repeater systems may have an effective coverage area of tens of square miles. It is possible to provide one or more satellite receiving stations (and a single high power transmitting site) if a somewhat larger coverage area is desired. However, some governmental entities and other public service trunking system users may require an RF communications coverage area of hundreds of square miles. In order to provide such very large coverage areas it is necessary to provide multiple RF repeater sites and to automatically coordinate all sites so that a radio transceiver located anywhere in the system coverage area may efficiently communicate in a trunked manner with other radio transceivers located anywhere in the system coverage area.
FIG. 1 is a schematic diagram of a simplified exemplary multiple-site trunked radio repeater system having three radio repeater (transmitting/receiving) sites S1, S2, and S3 providing communications to geographic areas A1, A2, and A3, respectively. Mobile or portable transceivers within area A1 transmit signals to and receive signals from site S1; transceivers within area A2 transmit signals to and receive signals transmitted by site S2; and transceivers within area A3 transmit signals to and receive signals transmitted by site S3. Each repeater site S1, S2, S3 includes a set of repeating transceivers operating on a control channel and plural RF working channels. Each site may typically have a central site controller (e.g., a digital computer) that acts as a central point for communications in the site, and is capable of functioning relatively autonomously if all participants of a call are located within its associated coverage area.
However, to enable communications from one area to another a switching network, as for example the assignee's "multisite switch" ("multisite coordinator") described herein, must be provided to establish audio and control signal pathways between repeaters of different sites. Moreover, such pathways must be set up at the beginning of each call and taken down at the end of each call. For example, the site controller (S1) receives a call from a mobile radio in A1 requesting a channel to communicate with a specific callee. A caller requests a channel simply by pressing the push-to-talk (PTT) button on his microphone. This informs the site controller S1 via an "inbound" digital control message transmitted over the RF control channel that a working or audio channel is requested. The site controller assigns a channel to the call and instructs the caller's radio unit to switch from the control channel to the audio channel assigned to the call. This assigned channel is applicable only within the area covered by the site.
In addition, the site controller sends the channel assignment to the multisite switch (200) which assigns an internal audio slot to the call. The multisite switch also sends a channel request over a control messaging bus to other site controllers having a designated callee within their site area. Audio signals are routed such that audio pathways are created to serve the callee(s) and one or more dispatcher consoles 202 involved in the communication. Upon receiving a channel request, these "secondary" site controllers (in the sense they did not originate the call) assign an RF working channel to the call. Each secondary channel is operative only in the area covered by the secondary site controller. The secondary site controller(s) also sends the channel assignment back up to the multisite switch.
Thus, the caller communicates with a unit (or a group of designated units referred to as a "talk group") in another area via the multisite switch. The call is initially transmitted to the primary site controller, routed through an assigned audio slot in the multisite switch, and retransmitted by the secondary sites on various assigned channels in those other areas. When the call ends, the primary site controller deactivates the assigned channel for that site and notifies multisite switch 200 that the call is terminated. The multisite switch propagates an end of call command ("channel drop") to all other site controllers. This releases all working channels assigned to the call and breaks the associated audio routing pathways.
The multisite switch has a distributed control architecture. The logical functions and computational workload of the multisite switch are shared by various distributed microprocessor "nodes". Each node is connected either to a site controller 102, dispatch console 202, public and/or private landline telephone exchanges and other components of the particular communications system. Most nodes function as switch interfaces and include, for example, Master Interface Modules (MIMs) for nodes coupled to site controllers and Console Interface Modules (CIMs) for nodes coupled to dispatch consoles. Each interface module is supported by a controller card that utilizes several microprocessors. All of the cards have substantially the same hardware and are interchangeable. Each card acts as a gateway interface into the distributed control switch network.
Detailed description and operation of the multisite switch, generally, is set forth in commonly assigned U.S. patent application Ser. No. 07/658,844 filed Feb. 22, 1991 entitled "Distributed Multisite Switch Architecture", the disclosure of which is also incorporated herein by reference.
In addition to providing communications between mobile radio units in different areas, multisite switch 200 provides communications between land-line telephone subscribers and radio units as well as dispatchers and mobile radio units. Land-line telephone subscribers can communicate with radio units by dialing an access number as well as a radio unit (or group) identification number which is routed to the trunked communications system through a central telephone interconnect switch (CTIS) 212 and multisite switch 200. One or more dispatch consoles 202 is connected to the multisite switch in the same manner as the site controllers 102. Both land-line subscribers and dispatch console operators can issue a channel call request through the multisite switch to a site controller 102 to call for example a mobile radio unit.
Each dispatch console 202 may participate in calls in its area. Thus, when a call comes through the multisite switch from another area to a mobile radio, the switch informs the dispatch console 202 of the call in addition to notifying the corresponding site controller 102. The dispatch operator can then listen or participate in the call. Multisite switch 200 also handles calls to groups ("talk groups") of mobile units and/or dispatch consoles by ensuring that the associated site controllers for all of the callees in a particular called group assign a working channel to the group call.
The present invention particularly relates to a distributed method of arbitrating talk group call contentions in a multisite system. Call contention in a multisite environment occurs when multiple callers at different sites attempt to transmit on a common talk group at nearly the same moment. The call processing system that coordinates communications between sites (site controllers) must resolve this contention to ensure that the transmission is processed consistently at all sites. On a multisite talk group it is unacceptable to have some of the listeners hear one transmitter while others are hearing something else.
Initial attempts to solve this problem included summing the audio from all transmitters at the multisite switch. This assures that transmissions are heard but results in potentially confusing audio transmissions with more than one voice being heard at a time. In addition, for voice guard calls summation of the transmissions from the radios is not feasible since it will interfere with encryption/decryption process of the call. Similarly, for digital data calls summation of multiple transmissions is unacceptable since it would invalidate the data from each transmitting station.
Conventionally, using a central arbitrator is one approach to solving contention problems. If all calls are single threaded at one point through a central arbitrator, the arbitrator can select the first transmitter, then deny other transmitters until the first caller is finished. However, the central arbitrator method results in at least the following significant problems when implemented in a distributed control call processing system such as a multisite switch: 1) call processing nodes in the system must communicate with the arbitration unit for each call, which adds a significant call processing overhead to each call whether call contention occurs or not; and, 2) single threading calls through a central arbitration routine creates a single point failure mode for the call processing system.
Accordingly, the present invention resolves talk group call contention between sites in a multisite system by utilizing distributed call arbitration processes to avoid the performance bottleneck and the "single point of failure" mode that would arise from using a conventional central arbitration scheme. More specifically, the present invention utilizes a distributed approach to contention arbitration wherein each site interface autonomously determines which call should be given priority in a contention situation based upon a predetermined common arbitration scheme. This scheme can most succinctly be described as "first call wins, tie goes to the highest numbered site." In addition, the arbitration method in accordance with the present invention provides for handling "emergency" calls by giving those calls priority over non-emergency calls.